Calibration of Theodolites

Theodolites are a type of optical instrument used for measuring precise horizontal and vertical angles. They are often used by land surveyors for determining potential building dimensions and property boundaries when sighting vacant land. They are also used extensively in the construction and engineering industries to layout building foundations and walls, leveling of lots and grade work, mapping out roadways, bridges and other infrastructure construction and other large-scale construction projects.

 

Theodolite Design

Theodolites consist of an optical sighting scope in a fork-mounted design that allows the scope to rotate freely around both the horizontal and vertical axes. Crosshairs located within the scope are used to sight and align the object being measured. Bubble levels are used for the proper leveling of the instrument.

There are two different ways theodolites measure the angles, by using graduated setting circles or by using internal optical scales and digital readouts. Digital-based theodolites are far more common, easier to use and more accurate than using graduated setting circles. Angles measured with digital theodolites can achieve accuracies down to a few seconds of arc.

Many theodolites today are known as ‘Total Stations’ where the instruments can measure the angles electronically. The digital information is stored in memory and the instrument can use the basic angle information to calculate distances from the instrument to a specific point and perform automatic triangulation calculations.

 

Theodolite Calibration

As with all precision measuring instruments, theodolites cannot perform accurate measurements unless they are calibrated periodically. At a typical construction site, Theodolites are subject to hazardous working conditions from temperature extremes, excessive vibrations from other construction equipment and they are often bumped or even dropped when being transported to the site.

Theodolites that are used frequently or if they have not been used for a significant amount of time can show measurement errors. Due to the high accuracy of theodolites, it will not be obvious to the user if the instrument is not reading correctly.

The proper calibration of theodolites can be performed in various ways. The actual procedure and equipment required for the calibration will depend on the type of the theodolite and the accuracies required.

 

Calibrating the horizontal scales of the theodolite involves comparing the instrument’s readout with a set of known horizontal angle values.

This can be performed outdoors where surveyor’s baselines are set up, where precise north/south/east/west lines are formed. Multiple autocollimators could also be setup in an exact quadrant system in a large enclosed area.

Inside a calibration laboratory, a single autocollimator and a precision rotary indexing table with a highly accurate encoder are often used. The theodolite is attached to the indexing table and zeroed while pointing to the autocollimator or other reference target. The indexing table is then turned to a specific angle, and the theodolite is turned to again align with the target. The readout on the theodolite should indicate the indication on the indexing table.

 

For the vertical angle calibration, the autocollimator and a precision rotary indexing table could also be used, however, a fixture will need to be built that holds the theodolite in a precise and balanced horizontal position.

There are large theodolite calibration stands that are often used for the vertical measurements. These stands contain several autocollimators set up in various vertical angle configurations, such as 15°, 30° or 45°, to accurately measure the theodolite’s vertical readout.

Another way is to use trigonometry calculations to determine the vertical angles. A fixture is used that consists of a long, movable track with precision rulers in both the vertical and horizontal planes. The theodolite would be referenced to a specific zero measurement value on the precision ruler and then moved vertically to an appropriate graduation on the ruler. The measurement of the distance from the theodolite to the ruler and the measurement of the height to the target graduation on the ruler would calculate the angle of measurement for the instrument.

In addition to the horizontal and vertical angle measurement accuracies, the proper calibration of a theodolite also involves checking the instrument’s leveling bubbles, crosshair alignment, and the runout of the horizontal and vertical travel. The instrument will also be cleaned and any broken or worn parts will be repaired or replaced.

can calibrate many types of theodolites and other optical equipment in our ISO/IEC 17025 accredited calibration laboratory. For more information on our equipment calibration capabilities, contact e2b calibration.

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